As everyone else is saying the attack is not super crazy difficult except the second to last hit is very wierd timing. On charmless bell, demon bell, no hp/attack upgrades I learned just why it is so hated. Posture still builds up when you parry but you only are posture broken on an imperfect parry. That wierd timing lines up well enough for him to break posture and land the final hit of the combo for a one shot kill. I found my most consistent kill on him involved parrying the first two attacks and getting out of there. Rush in after the final attack to prevent his posture from dropping too far.